OK ... I found a solution that worked for my situation, which is just like rlcarter's situation.
In my case, I changed skins from DNN-Blue/Horizontal/Fixed to DNN-Blue/Vertical/Full. This immediately caused my problem. To resolve this, I made a backup copy of the failing skin: "Vertical Menu - Full Width.ascx", found in "<web-root>\Portals\_default\Skins\DNN-Blue\". I then copied over the original skin file with the working skin: "Horizontal Menu - Fixed Width.ascx". The problem was immediately resolved.
However, I wish I knew the protocol for reporting a bug as this is a faily major bug, in two ways. One, a "factory-installed" skin should not crash your portal and everything else, including ability to log in as host. And two, ANY skin should not be allowed to crash any portal.
There should be some fall-back strategy for ensuring that site functionality is not impaired by faulty skins. If a skin fails to load properly, DNN should catch that particular class of exception and load a basic, robust, solid skin that is "guaranteed" to always work. With proper exception handling and better error reporting, this error could then be resolved through the interface instead of having to manually poke values in a DB or manipulating files via FTP.
How does one submit such feature request or bug reports?